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A spin-torque oscillator (STO) with perpendicularly magnetized reference layer and a field generating layer (FGL) with high saturation moment was compared with an STO having a perpendicular anisotropy layer (PL) on the FGL using micro-magnetic simulation. The STO without PL was more suitable for obtaining areal-density over 5 Tb/in2 from the viewpoint of achieving a high ac magnetic field and thin stack thickness required for an MAMR head. The FGL stayed in-plane against a strong trailing gap field, and stable high-frequency oscillation was achieved over 10 kOe of perpendicular external field. Magneto-resistance measurement with high applied current was carried out in order to distinguish the stable oscillation with large FGL oscillation trajectories. Qualitative correspondence between the simulation and the experimental results indicated that stable oscillation with large FGL oscillation trajectories was realized in the structure without PL.